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Originally Posted by Lil bastard
@JackG, You can use a DMM to test AC voltage output at the amp speaker terminals, or speaker lead wire. The reading (signal present - Amp ON) will be a positive number if the test leads are on like polarity, or it will read a negative number if you have positive lead on a negative terminal.
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LB, I have to apologize. I set you up a bit. In real life, I'm an EE (Electronic Engineer), and I was having a bit of trouble reconciling your "test".
You see, AC voltage (Alternating Current) has no polarity. A DMM on the AC scale will not show a + or -, as there is no polarity to indicate. It will simply indicate the AC voltage.
A DMM on the DC voltage scale *will* show polarity. If you measure a 9V battery, and the leads are plus to plus and minus to minus, the meter will indicate + 9V. If you swap the leads, it will indicate -9V.
The problem with your test is that an audio signal is AC. Voltage drives the speaker in an AC format, pushing and pulling the speaker cone in and out, producing the pressure waves our ears interpret as sound. In fact, you can connect a battery (a DC voltage) to a speaker's terminals and it will drive the speaker cone either out, or in. Reverse the battery, and the speaker cone will reverse it's travel. The AC voltage representation of sound moves the speaker cone in and out, making the sound waves we hear as music.
Speaker polarity simply makes sure that when the signal is "positive", all of the speaker cones move out. If some of the speakers are out of phase, then some move out and some move in (out of phase) and they cancel each other. If you connect *all* of the speakers out of phase, then the music will still be OK. What's important is that all of the speakers are connected in the same phase, with all of them moving in or out at the same time.
The bottom line is that your test isn't valid. While you may be able to measure some DC +/- component on some types of audio amplifiers, not all amps will be the same. If you connect a DMM set on the DC scale to an audio amp output, it will try to figure out if the signal is mostly positive or negative. That results will totally depend on what sound is playing at the moment you make the measurement. If it were a pure tone sine wave playing, the DC voltage would be 0, as the plus and minus voltages would be identical.
Without factory documentation or some sort of marking on the original speakers as to their polarity, the final test is going to have to be your ears. If the speakers are out of phase, you'll notice a lack of bass and a fuzzy stereo image. Flipping the wires +/- while listening to the same music at the same level will tell the tale.